In the art a number of different solutions to the construction of the magnet system have been suggested. When using magnet systems as drivers for generating the sound by moving the membrane it is customary to arrange a gap between two parts of the magnet system so that there will be a magnetic flux field arranged across this gap. In the gap is arranged a voice coil. The voice coil will move in the flux field in response to an alternating current induced in the coil. The magnetic flux field of the magnet will force the coil to move in the magnetic flux field substantially perpendicular to the direction of the flux lines making up the flux field and perpendicular to the direction of the current. The alternating current in the voice coil will when the voice coil is attached to a membrane generate the sound stemming from a loudspeaker.
In the art there are generally two types of magnet assembly designs, the first being overhung where a relatively wide voice coil is arranged in a relatively narrow gap in such a way that the actual extension of the coil exceeds the actual extension of the gap. The other principle commonly applied is a so-called underhung system where a relatively narrow coil is arranged in a relatively wide gap in such a way that the actual extension of the gap exceeds the actual extension of the voice coil.
The present invention is suitable with both types of designs as well as a neutral hung design, i.e. a design where the voice coil and the gap are of the same dimensions.
An example of a prior art loudspeaker assembly is disclosed in US 2002/0106101. This system comprises a driver unit comprising a central T-yoke around which a permanent magnet is arranged. The construction provides a gap in which the voice coil may move almost at the periphery of the driver. Furthermore, in order to save construction height, the driver is partly arranged in front of the loudspeaker membrane, which will give rise to sound distortion and a rather complicated design with respect to fastening of the driver to the chassis.
A further example of prior art design is known from WO 98/47312 wherein a magnet system is arranged in connection with a yoke construction. The gap in which the voice coil travels is arranged in the traditional manner as discussed above. Therefore, this construction also experiences problems resulting in distortion due to magnetic flux roll-off in either end (upper and lower) of the gap due to magnetic flux concentration in these areas. Where the difference in active cross-sectional area between the magnet and the gap is large, roll-off effects will be created.
Another prior art design is illustrated in DE 3108715. In this magnet assembly the magnet is particular in that the magnet surrounding the air gap is made from a composite material comprising iron powder with an artificial binder, typically based on a polymer. The purpose of using a composite conductive material is to minimize the eddy currents which will arise as the voice coil moves in the air gap due to the changes in the currents in the voice coil and the magnetic poles.
In general, it is desirable to obtain as linear a magnetic field across the air gap as possible in order to avoid distortion of the produced sound. The eddy currents will create distortion, and as such it is a desire to create a magnetic flux in the air gap which is substantially free of eddy currents.
This problem has also been addressed in JP 56-128099 and JP 59-21199 wherein iron powder has been sintered under pressure and high temperature, (approximately 1200° C.), in order to create a non-conductive material which in the above mentioned Japanese publications are arranged on either side of the air gap in order to minimize the effect of eddy currents. Although the resulting magnetic assembly is improved over the prior art and does reduce distortion, they still do not provide an optimum sound reproduction in that the effect of the eddy currents has a direct effect on the harmonic capabilities of the loudspeaker. The sintering process is made by using iron powder which together with the phosphor additive during the sintering process is melted together creating a relatively hard material which has an electrical conductivity approximately ten times less than that of normal iron. As such the magnetic conductivity is maintained whereas the electrical conductivity creating the eddy currents has been decreased whereby also the effect of the eddy currents is decreased.